The cut of a gem stone, such as a diamond, significantly affects its appearance and, in particular, the “brilliance” of the diamond. Features such as “brilliance” affect the commercial value of a diamond. Measures of the quality of a cut diamond include its proportions, symmetry and polish. Of these quality measures, only proportions have been routinely determined by automated means and graded, with acceptable accuracy, using calibrated apparatus in a repeatable and objective manner. Other measures, such as symmetry and polish, are still determined using manual methods, for example by an expert viewing the diamond through a microscope or other device for comparative symmetry grading purposes. Assessing these properties is thus a labour intensive task. Furthermore, these other quality measures therefore remain subjective and cannot be assessed in an objective manner that may be universally recognised.
In the past, attempts have been made to extend methods and apparatus designed for measuring diamond proportions in order to obtain additional information about the cut of the diamond. In principle, if sufficiently accurate measurements of the facet locations and dimensions can be obtained, a complete model, such as a three dimensional virtual wire-frame model, of the diamond can be constructed from which further properties, such as the symmetry of the diamond, can be determined. However, such attempts have proven inadequate. In particular, there has been no commercially available, automated and objective method for measuring the dimensions of a diamond with similar or greater accuracy as compared with the accuracy that can be achieved with manual gauges or micrometers. Furthermore, it has not been possible to measure facet angles to equivalent or greater accuracy than that with which the diamond can be cut using modern cutting tools and techniques. Thus there has been no objective method for distinguishing between diamonds of approximately the same quality of cut, even though an expert may be able to do so subjectively.
While there are computer aided methods being developed to rate the quality of cut of a diamond from a scanned wire frame model, the current accuracy of these methods is inadequate to provide a reliable indication of quality measures.
The most widely used apparatus and method for measuring the proportions of a cut diamond are illustrated in FIGS. 1A to 1C. For example, one commercial implementation of the method is that incorporated in the Sarin DiaMension proportion grading machine, although this is not the only such implementation. This prior art method is herein referred to as the “silhouette grading method”.
In the silhouette grading method, a diamond 102 is seated upon a turntable 104 which may be actuated by, for example, a precision stepper motor mechanism to rotate the diamond about a central axis. The diamond is lit from behind by a light source and collimating lens arrangement 106 and viewed from the front by a camera and focussing lens arrangement 108. From the viewpoint of the camera, the diamond is seen in silhouette 110, with only the outline visible, the entire body of the diamond appearing dark. Thus, it is not possible to identify, in the images captured by the camera, any information regarding the location or dimensions of any of the facets of the diamond e.g. facet 112. It will therefore be understood that the contrasting shading of the facet 112 shown in FIGS. 1A to 1C is for illustrative purposes only, to assist in identifying the facet in the drawings, and does not represent any actual visible feature of the diamond 102.
The only facets of the diamond about which any information is directly available from the viewpoint of the camera are those comprising the edges of the silhouette image e.g. facet 114. Considering, for example, a point 116 labelled “A” in the drawing along an edge of the side facet 114, it is possible to identify in the silhouette image the X and Z coordinates of the point 116. However, no information regarding the Y coordinate is available.
Thus, it can be understood that, using the silhouette grading method, incomplete information is available regarding the precise locations in three dimensions of facet edges and vertices. The information available consists of that which can be observed in the outline of the stone as it is rotated. This information includes the diamond proportions such as table width, crown height, crown angle, girdle height, pavilion depth and pavilion angle. However, further information regarding the facet locations and dimensions in three dimensions must be calculated from this incomplete set of measured properties of the diamond. Although such calculation may be possible, subject to assumptions regarding the cut of the diamond, in practice the accuracy with which facet locations can be determined is highly dependent upon the accuracy of the original measurements.
For example, if a facet such as facet 114 shown in FIG. 1 is, in fact, slightly rotated towards or away from the plane perpendicular to the camera, it will be appreciated that the measured angle of the facet will be subject to an error that will prevent the construction of an accurate wire-frame model.
In this regard, small errors in measurements of angles may result in a wire frame model of the diamond in which facet edges apparently do not meet correctly in a single vertex even when the measured diamond is almost perfect, and all edges in fact meet at the vertex. This problem is illustrated in FIG. 2, which shows a top view of a wire-frame model of a diamond obtained from the silhouette grading method. Whereas the real diamond may be perfectly cut, the model includes facet junctions that are not accurately determined. For example, the distinct junctions 202, 204 should in fact meet at a single point. Additionally, the distinct junctions 206, 208 should also meet at a single point, symmetrically placed with respect to the actual common meeting point of junctions 202, 204. Such errors make it impossible to assess the symmetry of the diamond by automated analysis of a model. While it is possible to employ computer algorithms to “close” the facet junctions on the wire-frame model, this method has the drawback that it is impossible to completely distinguish between measurement errors and actual instances of diamonds with poor facet meeting. Accordingly, such algorithms may result in a wire-frame model that has better symmetry than the original diamond.
As a result, it therefore remains necessary to employ the services of an expert human operator to examine every facet junction with the aid of a microscope. Improved accuracy of current wire frame modelling is also required for practical implementation of computerised methods for grading of cut quality.
FIG. 3 illustrates an apparatus that allows the silhouette grading method to be extended into a third dimension. In this apparatus, an additional camera and lens arrangement 302 is located above the diamond 102 in order to capture a top view of the stone. However, from the viewpoint of the additional camera 302, the diamond 102 is still seen only in silhouette, and there is therefore no further information available regarding the Y coordinate of the point “A” 116 which, when viewed from the top, is located within the dark silhouette image. This extended arrangement overcomes one problem of the single camera apparatus, in that it is now possible to identify indentations 117 in the girdle of the diamond 102 which are not visible from the viewpoint of the first camera. However, the arrangement offers no further benefit in obtaining the correct dimensions and locations of the facets of the diamond.
A further extension of the silhouette grading method, disclosed in U.S. Pat. No. 6,567,156, involves the use of a coherent collimated beam of laser light, directed towards a diamond at an angle of approximately 30 degrees to the horizontal. The image of an oblique laser beam as observed on the diamond facets is captured by the camera 108. The purpose of this extension to the method is to identify indentations in the stone, and especially in uncut and partly cut stones, that cannot be detected from the camera positions 108, and that are not always in view of the camera 302 in the previously described methods. However the method is, in fact, less accurate than the standard silhouette grading method in determining the proportions of the diamond, and is therefore less suited for accurately determining the locations of the facets.
It is therefore an object of the present invention to mitigate at least one of the abovementioned problems in the prior art, and in particular to provide an apparatus and method by which the location and dimensions of a facet of a diamond may be more accurately determined.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art on or before the priority date of the claims herein.